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September, 2010
MAINTENANCE MINUTE
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Welcome to the Centrifugal Blower Maintenance Minute,  
the 60 second newsletter designed specifically for centrifugal air & gas blower operators.  Each month we will feature a different aspect of service and maintenance to centrifugal blowers. We will review operation, troubleshooting, actual case histories, and answer reader questions.  
Bearing Failures
 

 

 

When your blower suddenly starts making more noise, most people assume the bearings must be going bad.  While there are certainly times when bearing noise is surely possible, there are a myriad of reasons for an increase in blower noise. Even I can't identify for sure what a particular noise might be just by listening to the blower in operation.  Oftentimes I have to remove a bearing cover to confirm the bearing has a broken cage, or is without grease, or whatever.  Only when you can hear the bearings running "rough", is it easy to identify bearing noise. 

 

Bearings start wearing out as soon as they are installed.  How long they last varies with the operation and maintenance. If maintained properly, the bearings in centrifugal blowers will last the life of the machine-20-30 years! I've seen blowers that run 24-7 and last for 15 or more years.  I've also had blowers that only run 20 - 30 hours per year, and they are probably still in operation now-40 years later.  Maintenance is the key to long bearing life.

 

Obviously the #1 cause of bearing failure is lubrication.  If it doesn't have enough grease, the metal to metal contact will lead to bearing failure.  And if it has too much grease, the bearing will run in a much hotter environment and thereby shorten its' life. Another concern is contamination with grease.  Whenever grease is added to a bearing, there is a possibility of contamination. Cleaning and re-packing the bearings, as some manufacturer's recommend, has an even greater possibility of contamination.  Any type of foreign material, or moisture, is considered contamination.  It only takes one spec of dirt/stone/metal chip to lodge between the bearing ball and the bearing raceway. 

 

The #2 cause of bearing failure is shaft misalignment.  The C3 clearance fit between the bearing balls and the inner & outer races of a bearing in a centrifugal blower is very small-less than .0005".  The bearings are designed to operate in exactly the same plane.  If you put a shaft thru the inner race and try to push or pull(twist) the inner race by moving the shaft, the inner race is not running true to the outer race.  The life of the bearing will be shortened.  Part of the alignment procedure is to correct for soft foot.  Soft foot distorts the outer race of the bearing when it pushes/pulls(twists) the main frame of the blower/motor housings.   Misalignment will not usually cause excessive vibration and noise until after the bearing becomes severely damaged, which could be years after startup. 

 

Recently I've seen more cases of bearing failures where the "cage" fails.  I am not sure if the cage failure is a result, or the cause, of the bearing failure.  For this reason, I do not use the least expensive bearing, but try to install well known brand bearings(SKF,FAG,MRC).

 

There are numerous other causes of bearing failures, such as surge, that are related to how the blower is operated, that I will save for another article.  Obviously it is best to replace the bearing before is suffers a catastrophic failure that damages the shaft and bearing housing. The best way to do that is with regular monitoring of the condition of the bearings along with trending of the overall vibration levels.  Monthly monitoring is the recommended interval, but even that may not be enough for some operations.  Some installations can be adequately covered with semi-annual visits to confirm there is no danger of imminent failure, but a bearing only needs a couple of weeks to go from good to bad.  Invest in PM, it can save you money.

 

� Copyright  2010 Snyder Technical Services, LLC.  All Rights Reserved


Maintenance Minute is published each month by Snyder Technical Services, LLC and is based on the work of former Lamson National Service Manager, Bill Snyder.  Should you have any questions, or problems to discuss, please contact me. 
 
Sincerely,
 
Bill Snyder
Founder - Snyder Technical  Services, LLC 
Next Month: 
 
Blower Condition Reporting    
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